1. Field of the Invention
The present invention relates generally to an optical storage medium such as an optical disc and to a manufacturing method for the same. More specifically, this invention relates to the structure of a high density, multiple layer optical storage medium having plural data recording layers with a guide groove, and to a manufacturing method for this optical storage medium.
2. Description of Related Art
Compact Discs (CDs) are now widely used and optical discs have earned an important role as a recording and storage medium. While conventional CD media is read-only, CD-R (recordable) and CD-RW (rewritable) media can also be used by the end user for recording and reproducing information. Subsequent research continues to develop other types of even higher density optical discs.
Optical discs have a guide groove consisting of a land and groove pattern for signal recording formed in the data recording layer. When seen from the side emitting the laser beam for recording or reproducing data, the side that is recessed in a concave shape away from the laser beam source is called the land, and the side that protrudes toward the laser beam source is called the groove. Recording capacity is determined in part by the pitch of the recordable and reproducible guide grooves (the track pitch). DVD-RAM media, for example, uses a land and groove recording method to record data to both lands and grooves. The track pitch is 0.74 μm in DVD-RAM media with a 2.6 GB recording capacity, and is 0.615 μm with a 4.7 GB recording capacity.
Optical disc recording density can be increased by increasing recording density in the recording layer and by increasing the number of recording layers. For example, Digital Versatile Disc (DVD) media, which has been standardized and are now readily available, is a read-only optical disc media with two data recording layers formed so that both data recording layers can be read from the same side of the disc. An optical disc having two data recording layers that can be recorded by an end user has also been developed and reported in the literature for use as recordable and readable DVD media.
The structure of a recordable and readable optical disc is described next with reference to FIG. 2. The recordable and readable optical disc shown in FIG. 2 has two data recording layers. Both data recording layers are made using a phase change recording material, a type of material whose optical characteristics can be changed between amorphous and crystalline states. Data is recorded to the data recording layers by recording a pattern of marks. More specifically, a first data recording layer 206 is formed in a first transparent layer 201 having a guide groove. The first data recording layer 206 consists of a recording film 203 made from a phase change material disposed between substantially transparent dielectric film 202 and substantially transparent dielectric film 204, and a translucent metal reflection film 205.
The second data recording layer 212 is formed in a second transparent layer 207 having a guide groove. This second data recording layer 212 consists of a metal reflection film 211 and a recording film 209 disposed between substantially transparent dielectric film 210 and substantially transparent dielectric film 208.
The first data recording layer 206 and second data recording layer 212 are bonded together separated a specific distance by an intervening substantially transparent adhesive layer 213. These data recording layers are formed following the guide groove patterns on the substrate surfaces, the recording film of each data recording layer is also formed following the guide groove pattern. The recording films therefore also have a guide groove.
The recording marks are formed in the recording film of the data recording layer, and the recording and reproducing characteristics are therefore more easily affected by the guide groove pattern on the recording film surface than the guide groove pattern on the substrate surface. In a conventional two-layer disc the guide groove width is wide compared to the thickness of the data recording layer, and two-layer optical discs are therefore produced according to the shape of the guide grooves formed on the substrate surface.
In order to further increase the recording density of the data recording layers, the guide groove pitch must be reduced and the guide grooves must be made accordingly narrower. However, if the ratio of the guide groove width to the data recording layer thickness (particularly the thickness of the film between the substrate surface and the recording film) is small, the width of the guide grooves appearing on the recording film surface when the recording film is formed will be small relative to the width of the guide grooves on the substrate surface. A difference also develops between the film thickness in the substrate guide grooves and the film formed between the guide grooves, which causes guide groove depth to change in addition to the guide groove width.
Because the configuration or thickness of each data recording layer differs in a multilayer optical disc, the desired performance cannot be achieved if the guide groove width and depth are the same in each layer even if the recording density is the same in each data recording layer.